The present disclosure relates to subject matter contained in Japanese Patent Application No. 5-325650 (filed on Dec. 24, 1993) and Japanese Patent Application No. 6-214481 (filed on Sep. 8, 1994), which are expressly incorporated herein by reference in their entireties.
1. Field of the Invention
The present invention relates to a video endoscope system designed so that the observation field of an endoscope is illuminated with illuminating light emitted from a light source and transmitted by a light guide, and an endoscope image signal in the observation field is taken out by a solid-state imaging device.
2. Description of the Prior Art
To obtain an observation image of good quality with a video endoscope system, it is necessary to optimize the brightness of illuminating light at all times.
Therefore, in a typical video endoscope system, the intensity of illuminating light is controlled by varying the voltage applied to the light source lamp, or controlling an aperture diaphragm which is provided in such a manner that it is movable into and out of an illuminating light path between the light source and the light guide, thereby enabling a favorable image brightness to be obtained at all times.
However, a change of the voltage applied to the light source lamp causes a change in the color temperature of the illuminating light. When the aperture diaphragm moves to change the position where it limits the cross-sectional area of the illuminating light path, spectral characteristics of illuminating light incident on the light guide from the light source change, resulting in a change of the color tone of the endoscope image.
Under these circumstances, the conventional practice is to correct the gain of each of the color image signals for three primary colors, i.e., red (R), green (G) and blue (B), according to the change of the light source voltage or the change in the aperture of the diaphragm, thereby making color tone correction of the endoscope image (see Japanese Patent Application Laid-Open (KOKAI) No. 2-181591).
However, more detailed analysis of the conventional technique has revealed that change in color tone of the endoscope image that occurs when the light source voltage is changed or when the aperture diaphragm is moved is not uniform throughout the screen image; there is a considerable difference in change of color tone between different portions of the screen image, e.g., between the central and peripheral portions of the screen image.
Such a phenomenon is attributable to the following reasons: The change in color temperature of the light source lamp that is caused when the light source voltage is changed differs between the central and peripheral portions of the screen image. The aperture diaphragm does not uniformly control the cross-sectional area of the illuminating light path but locally varies the cross-sectional area blocked by it. Therefore, the movement of the aperture diaphragm causes a change in the balance of light rays incident on the light guide at various angles. Thus, the above-described undesired phenomenon occurs on account of the light-transmitting characteristics of the light guide.
For the above reasons, color tone change occurring at every portion of the screen image is affected not only by the change of the light source voltage or the change in control condition of the aperture diaphragm but also by the characteristics of the light guide and the relationship between the light guide and the viewing optical system. That is, it also depends on the type of endoscope employed.